It has been expected that an aromatic polyimine is used as a heat-resistant material. As a process for preparing the aromatic polyimine, for instance, a method comprising subjecting an aromatic dialdehyde and an aromatic diamine to condensation has been studied. However, because an oligomer which is a resultant intermediate product during the condensation does not dissolve in most of solvents and the melting point of the oligomer is extremely high, it is difficult to polymerize the oligomer to have a higher molecular weight. It is reported that a polyimine having a high molecular weight can be obtained by selecting a phenolic solvent as a reaction solvent (Colloid and Polymer Science, vol. 261, p. 493 (1983)).
However, there is a great limitation with regard to productivity on an industrial scale because there are some disadvantages such that the phenolic solvent has high toxicity and pungency and that it is dangerous to handle the phenolic solvent, moreover because art industrial synthetic method of the aromatic dialdehyde has not yet been completed.
Also, there is proposed a process for preparing a polyimine having a high molecular weight comprising introducing a functional group into an aromatic dialdehyde or an aromatic diamine which is a raw material of the polyimine and increasing the solubility of a resulting oligomer, which is an intermediate product, in a solvent. For instance, as a polyimine having a phenolic hydroxyl group, there is known a condensation product of 5,5'-methylenebissalicylaldehyde and 1,2-diaminobenzene (Journal of Panerican Chemical Society, vol. 79, p. 6000 (1957)). However, because the solubility of an intermediate oligomer, which is generated during the condensation reaction, in the used solvent is not sufficiently increased, there is a disadvantage that the oligomer is easily extracted and precipitated at the time the molecular weight is reached about 10000, so that a product having a high molecular weight cannot be obtained. Therefore, the polyimine cannot be absolutely used as a polymeric material for industry.
Moreover, as a polyimine having a phenolic hydroxyl group, there is known a polyimine made by carrying out the condensation of glyoxal and 2,4-diaminophenol, and a polyimine having a high molecular weight is obtained (Macromolecule Chemie, vol. 56, p. 195 (1962)). However, because glyoxal which is an aliphatic dialdehyde is used as a dialdehyde component, the polyimine is wrong in heat resistance and cannot be used as a material for industry.
Thus, conventional polyimines containing a phenolic hydroxyl group are not satisfactory in terms of difficulty of their synthesis and heat resistance, and nowadays, it is earnestly desired that a polyimine having a high molecular weight, which is useful as a polymeric material for industry is developed.
In consideration of the above-mentioned actual circumstances, the present inventor has earnestly studied an aromatic polyimine having a phenolic hydroxyl group, which can be used as a heat-resistant polymeric material for industry and a process for preparing the same. As a result, the present inventor has found out that an aromatic polyimine made of an aromatic oligoimine composed of specific constituents can thoroughly solve the above-mentioned problems. The present invention has been accomplished based on the above knowledge.